1. Field of the Invention
The present invention relates to a lithographic apparatus, a method of generating a mask pattern, a device manufacturing method, a computer-readable storage medium storing a program for generating a mask pattern.
2. Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. The lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs), flat panel displays, and other devices involving fine structures. In a conventional lithographic apparatus, a patterning means, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC (or other device), and this pattern can be imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer or glass plate) that has a layer of radiation-sensitive material (e.g., resist). Instead of a mask, the patterning means may comprise an array of individually controllable elements that generate the circuit pattern.
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning” direction), while synchronously scanning the substrate parallel or anti-parallel to this direction.
In projection lithography the image printed on a substrate can be improved by adding optical proximity correction (OPC) features to the desired pattern. The OPC features are intended not to appear in the developed image themselves, but to affect the shape of the pattern features so that when developed the pattern features are closer to the desired pattern. They may also be used to bring the process windows of features of different pitches closer together to enable features of several different pitches to be printed in a single exposure. Known types of OPC features include scattering bars, serifs, hammerheads, and the like.
A binary mask (e.g., a chrome pattern on a quartz substrate) only allows for binary OPC features. To prevent the OPC features appearing in the developed pattern, they are made sub-resolution. For example, the OPC features are made to have a width less than a wavelength of exposure radiation in the case of scattering bars, so that the contrast in the projected image is less than a resist threshold. While known OPC features have been very successful in enabling features of critical dimension (CD) much less than the exposure wavelength (low k1) to be printed, further improvements would be desirable.
Therefore, what is needed is an improved method of providing Optical Proximity Correction features in lithography.